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Congenital defect is possible further evidence of inbreeding by Neanderthals at El Sidrón

High incidence of congenital clefts of the arch of the atlas observed among remains from Spanish site

The Atlas (C1) vertebra is the first cervical vertebra of the spine, immediately below the skull. It takes its name from the Greek Titan Atlas, who is popularly (but incorrectly) supposed to have held the world on his shoulders. Congenital defects of the anterior or posterior arches are rare in modern populations, occurring at frequencies of 0.087 to 0.1 percent and 0.73 to 3.84 percent respectively. The condition does not normally lead to clinical symptoms.

El Sidrón is a cave site in Asturias, northern Spain that has yielded extensive Neanderthal remains and stone tools since these were first discovered there in 1994. Over 2,400 human fossils have been recovered, representing at least thirteen individuals including seven adults, three adolescents, two juveniles and one infant. The remains are 49,000 years old. Ancient DNA has previously been obtained from the remains, indicating a small patrilocal (mature males remain within their family birth group, but females come from outside) group with low genetic diversity. Dental hypoplasias indicate that around half of the group members had experienced episodes of growth arrest due to malnutrition.

Researchers now report that two out of just three well-preserved atlases from the site present respectively a defect of the posterior arch and the anterior arch. Such a high incidence of a rare condition could be interpreted as further evidence of low genetic diversity of the group, and as a possible indicator of inbreeding. The picture that emerges from El Sidrón is of a small, barely-viable Neanderthal group struggling for survival in extremely harsh conditions.